Described herein are display mediums particularly suited for use in electrophoretic displays, for example in filling reservoirs or microcapsules of the electrophoretic display device. Also described are methods of displaying images with the electrophoretic display devices. Inclusion of a thermally reversible gelling agent in the display medium allows the image to retain stability even after an applied electrical field used in forming the image is removed.
Electrophoretic display devices, also referred to as electronic paper, are well known in the art. An electrophoretic display generally comprises a suspension of a single type of charged pigment particles colloidally dispersed in a colored liquid of matching specific gravity contained in a cell comprising two parallel and transparent conducting electrode panels. The charged particles are transported between the electrode panels under the influence of an electric field, and can therefore be made to display an image through appropriate application of the electric field on the electrodes. The advantages of electrophoretic displays as a means for providing information and displaying images has been well appreciated in the prior art.
U.S. Pat. No. 4,272,596, incorporated herein by reference in its entirety, illustrates a typical electrophoretic display structure. The electrophoretic display device comprises a pair of electrodes associated so as to form a cavity therebetween, which cavity is filled with an electrophoretic suspension layer comprising a colored suspending medium containing colloidal sized particles in an electrically insulating liquid and finely divided electrophoretic particles suspended in the suspending medium. A source of electrical potential is coupled to the electrodes. With an electric field applied, the particles form an image as they follow the field. See especially FIG. 1 therein.
U.S. Pat. No. 6,113,810, incorporated herein by reference, describes a dielectric dispersion for use in an electrophoretic display that includes a dielectric fluid, a first plurality of particles of a first color having a surface charge of a selected polarity dispersed within the dielectric fluid and a second plurality of particles of a second color having a surface charge of opposite polarity to that of the first plurality and a steric repulsion thereto preventing coagulation of the first and second pluralities. In one embodiment, the first and second plurality of particles are each formed by separate two stage dispersion polymerization reactions. Each set of particles is formed with unique secondary and functional monomers. Corresponding charge control agents are added to the dispersion to establish opposite polarities on the respective particles.
U.S. Pat. No. 6,017,584 discloses encapsulated displays, i.e., displays in which the electrophoretic fluid is encapsulated within a microcapsule. This fluid may be a mixture of two or more fluids or may be a single fluid. The displays may further comprise particles dispersed in a suspending fluid, wherein the particles contain a liquid. In either case, the suspending fluid may have a density or refractive index substantially matched to that of the particles dispersed therein. Application of electric fields to any of the electrophoretic displays described therein is indicated to affect an optical property of the display.
In one embodiment, the 584 Patent describes an encapsulated electro-osmotic display wherein a porous or gel-like internal phase of a capsule is swelled (i.e., filled) and drained by the electro-osmotically induced motion of a refractive index matching fluid (i.e., the difference between the refractive index of the fluid and the refractive index of the internal phase is preferably within 0.5). When the pores of the material are filled with the fluid, the capsule acts as a homogeneous optical material, thus largely transmitting or refracting light according to the bulk properties of the medium. When the pores are vacated by the mobile fluid, however, a larger quantity of optical index mismatches are present and light scattering is greatly increased. See column 10, lines 19-32 of the 584 Patent. This embodiment does not describe inclusion of a thermally reversible gelling agent as part of the suspending fluid.
U.S. Pat. No. 6,574,034, incorporated herein by reference, describes a method for displaying an image with an electrophoretic display device that includes a multiplicity of individual reservoirs or microcapsules, each containing an electrophoretic display fluid, located between two conductive film substrates, at least one of which is transparent. The method includes appropriately applying an electric field and a magnetic force to a selected individual reservoir in a manner to cause either a first set of particles or a second set of particles of the display fluid to be displayed The first set of particles exhibit a color different from and contrasting to a color of the second set of particles, and also preferably a different charging property.
One of the major problems still facing reimageable electronic paper is poor image stability, particularly after the electric field applied in forming the image in the display device is removed. Without a sustained electrostatic force, the electronic image degrades during handling (e.g., viewing/reading by a user). It is desired to develop an electrophoretic display having improved image stability.